Simulation of the Thermoforming Process of Glass Fiber Reinforced Polymeric Components: Investigation of the Combined Effect of the Crosshead Speed and Material Temperature

Thermoplastic based composite materials are increasingly gaining the interest of many engineering sectors, among them the automotive. Their unique features, resulted by the thermoplastic matrix characteristics, such as their recyclability and their formability have given new perspectives in their use. Among the most promising fabrication methods of thermoplastic composite components is the thermoforming process, the press forming of a heated semi-finalized composite plate. This method, although requires a quite simple working station and can be implemented in mass production, demonstrates a series of disadvantages on the quality of the product. Among them, the variation of the thickness, formation of wrinkles and overall undesired deformations are considered as defects that decrease the quality not only from the esthetical but also from the structural point of view. In the present work, a numerical analysis of the thermoforming process is conducted when applied to a box-shaped geometry. As an input for the material behavior during the process, mechanical tests are conducted at elevated temperatures. The flat and curved critical zones of the component are identified and an analysis of the effect of the temperature and the crosshead speed of the molds on the thickness distribution are examined as well as the overall residual stress field. The results indicate a strong dependency of the quality of the product by these parameters of the process.

Influence of Light-curing Distances on Microflexural Strength of Two Resin-based Composites

SUMMARY Objective: Our objective was to investigate the influence of different curing distances on microflexural strength and the microflexural modulus of two resin-based composites. Methods: Two nanohybrid composites were used; Filtek Z250 (Z250) and Tetric EvoCeram (TEC). Rectangular specimens were prepared (2-mm wide × 1-mm deep × 6-mm long) light cured according to the manufacturer's instructions at 0-mm, 2-mm, and 8-mm distances (n=10) and were stored wet at 37°C for 24 hours. A microflexural strength test was performed using a universal testing machine at a crosshead speed of 1 mm/min. The microflexural strength and microflexural modulus data were analyzed using a two-way analysis of variance followed by a Tukey multiple comparison post hoc test (α=0.05). Results: The TEC composite had a significantly higher microflexural strength at an 8-mm distance compared with the 0-mm distance. The Z250 composite expressed significantly higher microflexural strength, at 2-mm and 8-mm compared with the 0-mm distance. TEC showed a significantly higher microflexural modulus at an 8-mm distance compared with the 0-mm and 2-mm distances. Z250 also exhibited a significantly higher microflexural modulus at the 2-mm distance, compared with the 8-mm distance. In total, Z250 presented a significantly higher microflexural strength and modulus compared with TEC. Conclusion: Curing the explored composites at 2-mm or 8-mm distances from the specimen surface did not have a significant influence on microflexural strength but did significantly affect the microflexural modulus.

The impact of crosshead speed on the strength of spruce wood (Picea abies L.)

The effect of crosshead speed on the strength of spruce wood (Picea abies L.). This work examines the compression and tensile strength along the grain and bending perpendicularly to the grain of spruce wood was investigated at various crosshead speeds. The dependence of the immediate strength on the crosshead speed takes the form of an exponential function for compression and tension along the grain and bending perpendicular to the grain. The study showed that as the crosshead speed increases, the ultimate strength value increases regardless of the type of stress occurring. The bending strength is between the compression and tensile strength values. The strength for compression along the grain for the spruce wood tested is 50% of strength tension along grain length and 57% of the bending strength. The bending strength corresponds to 88% of the tensile strength.

Effect of Hydrofluoric Acid Concentration and Thermal Cycling on the Bond Strength of Brackets to Ceramic

This study to evaluate the effects of different hydrofluoric acid (HF) concentrations and thermal cycling on the shear bond strength (SBS) of brackets to ceramic. Cylinders of ceramic were divided into 10 groups (n=15), according to HF concentrations: 1-1%;2-2.5%;3-5%;4-7.5%;5-10% (storage 24 h); 6-1%;7-2.5%;8-5%;9-7.5%; and, 10-10% (thermal cycling). All cylinders were etched for 60s and received one layer of silane. Metallic brackets were bonded to the cylinders using Transbond-XT, light activated for 40 s, using a LED (Radii Plus) and stored in deionized water at 37o C for 24h. The groups 6 to 10 were submitted to thermal cycling (7,000 cycles - 5o/55oC). SBS was performed in an Instron at crosshead speed of 1.0 mm/min. Data were submitted to two-way ANOVA and Tukey’s post-hoc test (α=0.05). The Adhesive Remaining Index (ARI) was evaluated at 40x magnification. The different HF acid concentrations influenced on the SBS of the brackets to ceramic (p<0.05). The thermal cycling decreased the SBS of the brackets to ceramic for all acid concentrations (p<0.05). The ARI showed a predominance of scores 0 for all groups, with an increase in scores 1, 2 and 3 for the group storage for 24 h. In conclusion, the different HF acid concentrations 5.0%, 7.5% and 10% influenced on the SBS of brackets to ceramic. The thermal cycling decreased significantly the SBS of brackets to ceramic.

Comparative Time-Dependent Evaluation of Dimensional Accuracy of Dies Using Different Tray Adhesives: An In Vitro Study

Statement of Problem The adhesion of impression material to impression tray is very important. Tray adhesive plays a major role in making accurate impression. Although manufactures recommend the use of particular tray adhesives, comparison of their affective adhesiveness has not been reported. The effect of use of tray adhesives on dimensional accuracy of dies has not been established. Purpose The aim of this study was to compare the dimensional accuracy of dies using different tray adhesives at different time intervals. Materials and Methods First part of study comprised 120 samples in six groups with 20 samples in each group. First group comprised samples with no tray adhesive, and in other five groups, different types of tray adhesives were applied. The dies obtained were evaluated for upper diameter, lower diameter, and occlusogingival height. In the second part, there were a total of 125 samples in five groups with 25 samples in each group. Five different types of tray adhesive were applied for five different time intervals 5, 10, 15, 20, and 25 minutes, respectively. The specimens were tested in tensile mode for its debonding force at a crosshead speed of 5 mm/min, until separation failure occurred. Results Significant difference was seen for upper and lower diameters when compared with the group without any tray adhesive. The maximum bond strength was found in the group in which tray adhesive was applied for 20 minutes.

Compressive strength of calcium silicate-based cement

Summary Introduction The aim of this study was to compare compressive strength (Cs) of new nanostructural calcium silicate based cement (nCS) with commercial calcium silicate cement and conventional GIC. Methods Four nanostructural materials were tested: nanostructural calcium silicate based cement (nCS) (Jokanović et al.), MTA Plus (Cerkamed, Poland), Fuji IX (GC Corporation, Japan) and Ketac Universal Aplicap (3M ESPE, USA). Five samples of each material were mixed in accordance with manifecturer’s guidelines and positioned in metal moulds (ϕ4mm and 6mm). Compressive strength (Cs) expressed in MPa was determined after 24 hours, 7 days and 28 days respectively. Measurements were performed on universal testing equipment (Tinius Olsen, USA) at a crosshead speed of 1mm/min. For processing the results one-way ANOVA and post-hoc test were used. Results The highest values of compressive strength after 24h was found in conventional GIC Fuji IX (mean 38.56±13.31) and Ketac Universal (mean 40.77±7.96). Calcium silicate cements after 24h showed low values of compressive strength (MTA Plus 5.91±0.28 MPa, nCS 1.35±0.36 MPa). After 7 days, FUJI IX 47.42±9.33 MPa and Ketac Universal 35.25±10.60 MPa showed higher value of compressive strength than MTA Plus (15.09±2.77 MPa) and nCS (11.06±0.88 MPa). After 28 days the Cs value for conventional GIC Fuji IX was 48.03±7.82 MPa and Ketac Universal 36.65±11.13 MPa while for calcium silicate cements it was 16.47±1.89 MPa and nCS 14.39±1.63 MPa. There was statistically significant difference (p<0.05) in Cs between conventional GIC and CS cements after 24h, 7 and 28 days. Conclusions Calcium silicate cements initially showed lower values of compressive strength than conventional GIC that increased over time.

The effect of environment (Dry and Natural Saliva) on clasp retention: In vitro study - Part I

ABSTRACT Objective: The purpose of this study was to investigate the importance of environments (dry and wet) to dislodge the clasp. Materials and Methods: Mandibular test models with natural premolar and molar teeth were used to test four types of clasp (each 12) (Akers, rest plate Akers [RPA], half and half [H-H], and ring clasp) in dry and natural fresh saliva environments. Each clasp was pulled out 10 times with a crosshead speed of 10 mm/min and the force required to withdraw each was measured. Statistical Analysis Used: A paired sample t-test and Wilcoxon test were used. Results: There were significant differences between the dry and wet (natural fresh saliva) environment. However, while the mean of the environment for RPA and ring clasp type was significantly different, the H-H and Akers clasp type was not. Conclusion: The environment has an effect on dislodging the clasp but differs according to the type of clasp.

Effect of clasp type and pullout location on clasp retention in different environment: In vitro study

ABSTRACT Objective: The purpose of this study was to investigate the importance of pullout location and clasp types in two different environments to dislodge the clasp. Materials and Methods: Mandibular test models with natural premolars and molar teeth were used to test four types of clasp (each 12) (Akers, Rest plate Akers, Half and Half, and Ring clasp) with three different pullout location for each type (ring on the rest, loop on the saddle, and wax arising from both rests) in dry and natural fresh saliva environment. Each clasp was pulled out 10 times with a crosshead speed of 10 mm/min, and the force required to withdraw each was measured. Statistical Analysis Used: A one-way ANOVA and Tukey test were used. Results: The ring on the saddle pullout location has the highest retention force while ring on the rest was the lowest. In addition, ring clasp has the highest retention force. Conclusion: Clasp type and pullout location had a significant effect on the retentive force.